The present invention relates to the field of bearing measurement of wideband electromagnetic emitters, used in communication radar and electronic warfare systems
Systems involved in reception of electromagnetic signals, such as electronic warfare systems, radar systems and communication systems, often are required to locate the bearing of an electromagnetic source with high accuracy and high probability of intercept. Many systems are known in the art for performing this task, using different methods. One of the most advanced methods is known as xe2x80x9cmultimodal interferometryxe2x80x9d or xe2x80x9ccircular interferometryxe2x80x9d. The basic idea started with the xe2x80x9cAdcock bearing measurementxe2x80x9d, described in UK Patent No. 130490. This is basically a four element form of the circular interferometer. The principle of the Adcock bearing measurement has been expanded to any number of elements, with practical system implementations described, for example, in IL Patent No. 57910; and also by P. M. Eyring, xe2x80x9cCompact DF antenna delivers high AOA accuracyxe2x80x9d, Microwave and RF, June 1997; and by Murthy et al., xe2x80x9cModels simulate Butler-matirix-based DBDsxe2x80x9d, Microwave and RF, June 1996. Some expansions of the theory of phase modes have been published. See for example: B. Shellig, xe2x80x9cA matrix-fed circular array for continuous scanningxe2x80x9d, Proceedings of the IEEE, November 1968, pp. 2016-2027; D. E. N. Davis et al., xe2x80x9cAn adaptive circular array for HF direction finding and null steeringxe2x80x9d, IEEE LCAP 1985 (Conf. Proc. 248); J. R. F. Guy and D. E. N. Davies, xe2x80x9cStudies of the Adcock direction finder in terms of phase-mode excitations around circular arraysxe2x80x9d, The Radio and Electronic Engineer, vol. 53 no. 1, January 1983; and R. Eiges and H. D. Griffiths, xe2x80x9cMode-space spatial spectral estimation for circular arraysxe2x80x9d, IEEE Proc.xe2x80x94Radar, Sonar Navig., vol. 141 no. 6, December 1994. All of these expansions suffer from severe practical implementation difficulties.
The concept of using phase modes in a bearing measurement system now will be described briefly, referring to FIG. 1. An aggregate of receiving antenna elements 1 are equispaced around a circle. This aggregate feeds a RF Butler matrix 2 which is a modal-beam-forming netwvork that includes RF hybrid combiners and phase shifters, interconnected by RF transmission lines, as described, for example, in T. Macnamara, xe2x80x9cSimplified design procedures for Butler matrices incorporating 90xc2x0 hybrids or 180xc2x0 hybridsxe2x80x9d, IEEE Proc., vol. 134 part H no. 1, February 1987. The function of RF Butler matrix 2 is to phase-shift and sum the signals from all antenna elements 1 into several outputs, which are called modal beams. For modal beam numbered M in an array consisting of N elements 1, the weighted sum is:       S    ⁡          (      M      )        =            -              1                  N                      ⁢                  ∑                  i          =          0                          N          -          1                    ⁢              xe2x80x83            ⁢                        s          ⁡                      (            i            )                          ⁢                  exp          ⁡                      (                                          2                ⁢                π                ⁢                                  xe2x80x83                                ⁢                i                ⁢                                  xe2x80x83                                ⁢                M                            N                        )                              
where s(i) is the signal out of element i and S(M) is the signal out of modal beam M. This operation, which is equivalent to a Discrete Fourier Transform, is performed in the RF domain.
The modal beams have the following two major properties:
1. Each beam has a quasi-omnidirectional amplitude pattern, which means that the signal strength does not depend on the azimuth angle of the source.
2. The phase of modal beam M, xcex8(M), is quasi-proportional to the azimuth angle xcfx86 of the source, with M being the constant of proportioinality:
xcex8(M)xe2x89xa1Mxcfx86
Thus, by measuring the modal phases, it is straightforward to extract an estimate of the azimuth angle of the source. The higher the mode, the higher the accuracy; but measurements of lower order modes is required in order to resolve ambiguities occurring in the higher order modes. The inherent circular symmetry of the phase modes concept lends itself to most compact, minimal channel count designs, with minimal frequency, polarization and elevation dependence.
The modal beams from RF Butler matrix 2 are input to a set of multichannel receivers 3. Phase measurements of the modal beams have been implemented using several types of multichannel receivers 3, all tackling the task of amplification, dynamic range handling and correlating (phase measurement). Finally, the outputs of multichannel receivers 3 are input to a digital processing unit 4 that carries out digital signal processing to extract the bearing of the received signal. The types of receivers 3 that have been used in practical systems include:
1. Direct broadband receivers, with broadband limiting amplifiers and broadband correlators.
2. Homodyne receivers of several implementations, for example the implementation taught by Manuel in U.S. Pat. No. 5,661,485. These receivers downconvert all the channels to a single tone Intermediate Frequency (IF) signal, using one of the modal beams as a reference local Oscillator (LO), after the proper frequency shift. The dynamic ranging and the correlation between modal beams is performed in IF.
3. Super heterodyne receivers, which downconvert a predetermined portion of the spectrum into IF signals and perform the correlation in IF.
All of the prior art implementations use RF Butler matrix 2 as the spatial processor that produces the modal beams in the RF domain, the shortcomings of this approach stem from the limitations imposed by RF Butler matrix 2:
1. RF Butler matrix 2 has a considerable insertion loss (5 to 8 dB at 18 Ghz). This reduces the system""s sensitivity. Using a low noise preamplifier on each antenna element 1 alleviates this problem, at the expense of complexity, cost and also reduced accuracy, because of the transmission mismatch between amplifiers, which is hard to calibrate.
2. RF Butler matrix 2 has inherent inaccuracies which are hard to control, especially at the broadband and high frequencies often encountered in practical systems. These inaccuracies transform into deviations from the ideal phase-azimuth relationship, generating large bearing estimation errors. While sonic of these errors may be linearized, not all of these errors can be corrected; and the overall system performance is very sensitive to measurement errors, multipath, multisignals and reflections.
3. RF Butler matrix 2 is a very complicated RF supercomponent, imposing realization difficulties, high cost and mechanical constraints.
4. Advanced bearing algorithms for implementing null steering, multisource discrimination, etc., can not be used in practice because of the limited accuracy and the practically available phase modes from RF Butler matrix 2.
There is thus a widely recognized need for, and it would be highly advantageous to have, a device and method for broadband reception and bearing measurement of RF sources that does not employ a very difficult, highly specialized RF beamforming supercomponent such as RF Butler matrix 2.
The present invention is an innovative device and method for circular interferometry. The present invention eliminates the drawbacks and limitations of the RF processing unit used in the prior art by replacing the RF processing performed by this unit with digital or IF processing.
According to the present invention there is provided a device for determining a bearing of an incoming RF signal, including: (a) a plurality of primary antenna elements for receiving the RF signal; (b) for each primary antenna, a receiving channel for downconverting the received RF signal to a respective IF signal, each IF signal having a respective amplitude and a respective phase; and (c) a processing mechanism for inferring the bearing from the amplitudes and the phases.
Preferably, the primary antenna elements are equally spaced around at least a portion of a circle. Preferably, each primary antenna element is provided with an amplifier for amplifying the received RF signal.
Preferably, the receiving channels are homodyne receivers or heterodyne receivers.
Preferably, the device further includes a source of a local oscillator signal. The oscillator signal is introduced to each receiving channel via a port and is mixed with the received RF signal to produce the IF signal. In one embodiment of the device, the source includes a complex weighting combiner and, for each primary antenna element, a power splitter for diverting a portion of the received RF signal to the complex weighting combiner. One embodiment of the complex weighting combiner includes, for each primary antenna, a respective I/Q modulator, Another embodiment of the complex weighting combiner includes, for each primary antenna, a respective phase shifter and attenuator. In a second embodiment of the device, the source includes at least one auxiliary antenna for receiving the RF signal. In a third embodiment of the device, the source includes a tunable oscillator.
Preferably, the device further includes a distribution mechanism for distributing the local oscillator signal to the ports of the receiving channels. Preferably, the distribution mechanism also conditions the local oscillator signal, specifically, by amplifying the local oscillator signal, by amplitude-limiting the local oscillator signal, or by frequency shifting the local oscillator signal. Preferably, the distribution mechanism also filters the local oscillator signal.
According to one embodiment of the device, the processing mechanims is digital, and includes a digitizer for each receiving channel, and also a digital signal processor for computing the bearing from the digitized signals. According to another embodiment of the device, the processing mechanism is analog, and includes components such as hybrid networks, phase shifters, attenuators and I/Q modulators. Preferably, the analog processing mechanism includes an IF Butler matrix.
According to the present invention there is provided a method of determining a bearing of an incoming RF signal, including the steps of: (a) receiving the incoming RF signal, at a plurality of primary antenna elements, as a corresponding plurality of received RF signals; (b) downconverting the received RF signals to corresponding IF signals, each IF signal having a respective amplitude and phase; and (c) inferring the bearing from the amplitudes and the phases.
Preferably, the received RF signals are amplified prior to downconversion.
Preferably, the downconverting is effected by mixing each received RF signal with a local oscillator signal. There are three preferred methods of providing candidate signals to be used as the local oscillator signal. The first method is to divert a portion of the received RF signals and to combine the diverted RF signals. The second method is to use the incoming RF signal itself, as received by an auxiliary antenna. The third method is to use a tunable oscillator. The local oscillator signal actually used may be obtained by switching among two or among all three of these candidates, or alternatively by multiplexing two or all three of these candidates. Preferably, the local oscillator signal is conditioned and/or filtered prior to mixing. Examples of conditioning the local oscillator signal include amplifying the local oscillator signal, amplitude-limiting the local oscillator signal, and frequency shifting the local oscillator signal.
One preferred method of inferring the bearing is digital. The IF signals are digitized and the digitized signals are processed digitally. Preferably, the digital processing includes beam shaping. Preferably, the digital processing includes emulating a Butler matrix. Another preferred method of inferring the bearing is analog.
Preferably, the downconverting is effected using, for each received RF signal, a corresponding receiving channel. Preferably, the receiving channels are calibrated by injecting therein a calibration signal. Preferably, the calibration signal is injected at a common reference plane.
The present invention solves the problems imposed by RF Butler matrix 2 on the performance of a multi-modal circular interferometer by eliminating RF Butler matrix 2 altogether. The mathematical functions of RF Butler matrix 2 are performed either digitally or in IF.
A common signal derived either from an auxiliary external antenna of any type, or from the weighted sum of all the primary antenna elements, is used as a Shared Local Oscillator Signal (SLOS). This SLOS is used in a multichannel homodyne receiver of any type to downconvert the signals from all the primary antenna elements into single tone IF signals having the same phase and amplitude properties as the respective elements. These IF signals may be processed with an analog IF processor that performs the equivalent function of RF Butler matrix 2. Alternatively, and preferably, the IF signals are digitized into complex digital representations thereof, and the mathematical function of RF Butler matrix 2, and also any related algorithms, are performed in a Digital Signal Processing Unit. By substituting an external LO signal, derived from any type of oscillator, for the SLOS, a superheterodyne frequency-scanning receiver is implemented, using most of the components of the SLOS embodiment.
The improved performance of the present invention over the prior art includes:
1. Better system sensitivity, due to elimination of the losses associated with Rf Butler matrix 2.
2. Better bearing measurement accuracy and stability, due to elimination of errors introduced by RF Butler matrix 2.
3. Reduced system complexity and enhanced system modularity, due to elimination of RF Butler matrix 2.
4. Implementability of other spatial filtering algorithms, by improved accuracy and availability of all the signals.
The homodyne embodiment of the present invention is superficially similar to a circular interferometer based on the phase interferometer described in the above-referenced U.S. Pat. No. 5,661,485. The principal difference between the present invention and this prior art circular interferometer is that this prior art interferometer effects spatial processing of the received RF signals using an RF spatial processor such as RF Butler matrix 2, downconverts the spatially processed RF signals to IF, and infers the bearing of the incoming RF signal from only the phases of the IF signals, without regard to the amplitudes of the IF signals; whereas the present invention downconverts the received RF signals to IF with no intermediate RF spatial processing and then infers the bearing of the incoming RF signal from both the amplitudes and the phases of the IF signals.